Checking and setting proper crankshaft end play is a vital engine-building step. We show you how to do the job in this tech segment.

Engine building isn’t difficult – as long as everything goes according to plan. The challenge for the engine builder is to anticipate problems before they occur. Much of the process of becoming a successful engine builder is checking all the clearances and custom setting them when they are out of tolerance.

With the crank in place, set the main caps and lightly tighten to main cap bolts to around 10 to 20 ft-lbs of torque. Then tap the rear of the crankshaft forward with a soft mallet to align the paired thrust surfaces. Now the thrust main cap can be tightened to its torque spec. Do this in at least two steps to gradually load the cap.

Crankshafts are generally among the most abused components in an engine. One way to minimize this abuse and maximize the crankshaft’s opportunity to deliver a long service life is to make sure all the clearances are correct. For this story, we’ll be looking at thrust clearance or what is often called end play. This is the amount of clearance between the crankshaft’s thrust plate and the vertical surface of the main thrust bearing.

Mount a magnetic base to the engine and set the dial indicator to read off the crank snout. Gently pry the crank all the way forward and zero the gauge. The crank should move with very little effort and the dial indicator will read the clearance. In this case, we only have 0.002-inch so the thrust will need massaging.

It’s worthwhile to discuss first why it’s important to have a thrust bearing. There are transmission loads that tend to force the crankshaft forward. With automatic transmissions, this can originate from the torque converter. This should never exceed a light forward pressure, but this load does exist and must be accommodated.

Engine

Crankshaft End Play

Clearance (inches)

Small-block Chevy

0.003 - 0.011

Big-Block Chevy

0.006 - 0.010

GM LS Gen III / IV

0.0015- 0.0078

302-351W Ford

0.004 - 0.008

429 – 460 Ford

0.004 - 0.008

Ford Modular 5.0L

0.004 - 0.008

340 – 360 Mopar

0.002 - 0.007

440 Mopar

0.003 - 0.007

Mopar Gen III hemi

0.002 - 0.011

Ideal clearance would be mid-point between these minimum and maximum clearances.

Manual transmission thrust can be excessive with pressure plates that generate high static loads. The most abusive of these are the three-finger style pressure plates that use internal coil springs. With the clutch pedal on the floor, a major portion of the load released by the clutch pedal is directed forward into the crankshaft. These pressure plates are most often used in race engines, explaining why it’s always best to start an engine with the transmission in neutral so that the crank spins with no forward load. Starting a cold engine (when most of the oil has drained from this area) with the clutch pedal on the floor places tremendous load on the thrust bearing. It’s best to avoid this by starting the engine with the transmission in neutral.

Clamp the bearings together with a hose clamp so both can be adjusted simultaneously. We measure the thickness of the clamped thrust bearings with a dial caliper across several places on the thrust and record the highest spot. Using fine grit sandpaper and a piece of class, gently sand the bearings until the measurements indicate a proper clearance.

For this checking example, we will be using a K1 steel crankshaft in a Dart Little M cast iron small-block Chevy. It’s always best to test fit all clearances for a new engine before final installation in case modifications are necessary. For this application, we pre-assembled the rear main thrust along with the Number One main bearing, dropped in the crankshaft, and installed the main caps with the studs lightly tightened.

Before fully torquing the main studs, it’s necessary to align the two pieces of the thrust bearing. To do this lightly hit on the rear of the crank with a rubber or plastic mallet. This will ensure the thrust surfaces are even from the rear which is where all the force will originate. This ensures the paired bearings are parallel. With this accomplished, the main caps can be torqued to the proper spec.

We like to use a full sheet of 400 to 600 grit wet/dry sandpaper with a few drops of light machine oil like Marvel Mystery Oil to lubricate the process.

Next, you will need a magnetic base and dial indicator. Align the dial indicator plunger parallel to the crank snout and lightly force the crank backward and zero the dial indicator. Now lightly force the crank forward and read the amount of movement on the dial indicator. Different engines demand varying specs. Generally speaking, keeping the thrust clearance at 0.004 to 0.005-inch is appropriate but it is best to check the recommended clearance. For example, late model engines prefer a slightly tighter clearance to minimize travel of the crank sensor reluctor wheel. We’ve included a chart listing factory endplay dimensions for some of the more popular performance engines.

Always clean the bearing thoroughly with hot soapy water to make sure all the sanding grit is removed. We follow this by cleaning a second time with a paper towel and rubbing alcohol to make sure the bearing is clean.

If when assembling an engine you discover the clearance to be very tight, there is a simple way to increase the clearance. The generally accepted procedure is to clamp the two thrust bearings together with a hose clamp, making sure the thrust surfaces are aligned and flat. Also make sure the two halves are clamped as they sit in the engine – it’s possible to orient them incorrectly which will not produce the results you desire. Always position the two halves with the locating notches facing each other. Then place a full size sheet of 600-grit wet/dry sandpaper on a large plate of either plate glass or flat metal plate. Add a few drops of machine oil like Marvel Mystery oil to the sandpaper.

All Ford engines and the new generation LS small-block place the thrust bearing in the center main cap, which may be an advantage under high load conditions to stabilize the crankshaft.

It’s best to sand only the leading edge side of the thrust bearing when increasing clearance. This way, the thickest portion will be the trailing side which is where any wear will occur. Measure the combined width of the thrust bearing across both wear surfaces with either a quality dial caliper or a micrometer. We generally see a slight difference in thickness of perhaps 0.001-inch across the thrust bearing face.

Record this dimension and then keep sanding until you gain the necessary clearance. Generally you may need to only increase the clearance by 0.002- or 0.003-inch, but you will be surprised at how much sanding this will require. Some engine builders will lightly dress the sanded face with 1000 grit paper to polish the surface once the proper clearance is achieved. Of course, a thorough cleaning with hot soapy water and a sponge followed by a wipe-down with rubbing alcohol and a white paper towel is necessary to ensure that all of the sanding grit is removed before the bearing is re-inserted into the engine to recheck the clearance.

Installing the balancer properly also falls under the area of taking care of the thrust bearing. Use the proper tool like this one to press the balancer in place. Using a hammer to bear the balancer over the crank snout should be considered thrust bearing abuse.

Crankshafts that are discovered suffering from excessive thrust clearance are rare, assuming no damage has occurred to the crankshaft. Alternatives may be to try a different bearing manufacturer to see if the clearance will improve although this is unlikely. The only other solution is to have the crankshaft repaired to put the thrust thickness back to its stock thickness. This may cost nearly as much as the price of a new crankshaft.

Checking and setting clearances is all about stacking the longevity odds in your favor. The payoff is when that engine starts and runs properly and delivers a long, productive, and powerful life.